The present invention relates to an antifouling coating composition, and more particularly to an antifouling coating composition for use with underwater structures such as ships, port facilities, buoys, pipelines, bridges, submarine stations, submarine oil field excavation facilities, water conduit raceway tubes in power plants, cultivating fishing nets, stationary fishing nets. Such coating composition is suitable for preventing underwater living things from adhering and growing on the surface of the underwater structure.
Biofouling, the growth of barnacles, seaweeds, tubeworms and other marine organisms on the hulls of ocean-going vessels, and other underwater structures, cause the international marine community billions of dollars a year. In the case of ocean-going vessels, most of this money goes for the extra fuel needed to overcome the increased drag on vessels. Some of it is spent for hull cleaning and repainting and for the upkeep on propulsion equipment. Of the total amount of money, a tiny amount is invested in the search for better antifouling inhibitors.
One currently used hull antifouling coating contains species such as tributyltin compounds or copper oxide and function through leaching of the toxicant into the marine environment. The resulting environmental hazards of introducing such toxicants into the marine ecosystem include disruption of natural ecocycles for many commercially important shellfish and pollution of entire food chains. The removal and disposal of toxicant-containing coatings from ships and other structures also pose separate environmental hazards, driving up the cost of refurbishment.
An alternative approach is to use acrylic acid monomer compositions which are water soluble, i.e., a polyester resin with an acrylic acid group. Such coatings include a biocide, which after a certain amount of time becomes inactive. The composition, because it is water soluble, wears off over time, i.e., is ablative, and exposes new and active biocide at the surface. Such a composition is known as a self-polishing composition. Thus the alternative approach has been to employ a polymeric coating to function as a fouling release coating. Poly(dimethylsiloxane) (PDMS)based coatings have properties which meet some of those requirements. On the other hand, as noted, studies have shown that such a cured PDMS material becomes unstable when immersed in water for three months.
Another specific approach involves the use of a composition containing a majority by weight as resin-solid content a reaction-curable silicone resin composition, a silicone resin having the specific average molecular weight and viscosity and an alkoxy group at its molecular terminal. While such a composition exhibits non-toxic characteristics, it is silicone based and in addition to the discussed disadvantages, subject to premature wear requiring frequent maintenance in the form of reapplication of the coating.
In accordance with the invention, an antifouling coating composition, which is a silicone modified glass, is provided which is extremely effective in preventing fouling, and which is highly durable over time.
The antifouling composition of the present invention compromises a glassy matrix formed by crosslinking a mixture of a silanol-terminated silicone and an alkoxy functionalized siloxane to provide an interpenetrating polymer network of glass and silicone and at least two materials capable of microphase separation, at least one of which is graftable to the glassy matrix.
The present invention also provides a method of treating a substrate to prevent fouling thereof. The method includes applying to the substrate a mixture of a silanol-terminated silicone and an alkoxy functionalized siloxane, and at least two materials capable of microphase separation. Thereafter crosslinking the mixture to provide an interpenetrating polymer network of glass and silicone to which is grafted at least one of the materials capable of microphase separation.
Optionally the mixture/antifouling composition can include an agent capable of preventing or inhibiting slime (e.g., algae, bacteria, protozoa, diatoms, etc.) from growing on the surface of the coating. While in most cases, such agent will be included in the composition, there are instances when slime is not an issue, and the anti-slime agent can be omitted. Suitable agents capable of preventing or inhibiting slime include surfactants, emulsifiers, enzymes, silver compounds, quaternary amine compounds, sulfa-based antimicrobial compounds, saponin and cholesterol, and mixtures and blends thereof.
The present invention will be more fully understood by reference to the following description and examples. Variations and modifications of the embodiments of the invention can be substituted without departing from the principles of the invention, as will be evident to those skilled in the art.
The present invention is based on the discovery that a modified glassy matrix can be combined with at least two materials capable of microphase separation to make a uniform and tough antifouling coating for use on surfaces, particularly marine surfaces in an underwater environment.
The glassy matrix serves to provide a carrier or support material composition. The matrix provides critical properties such as good adhesion to the substrate on which the formulation is applied as a coating, toughness, crack resistance, durability, abrasion resistance and stability in an aqueous environment. The glassy matrix is formed by crosslinking a mixture of a silanol-terminated silicone and alkoxy functionalized siloxane to provide a silicate glass. Typically, the glassy matrix is crosslinked using an organotitanate or tin catalyst agent.
Suitable functionally-terminated silicones include polydimethylsiloxane silanol terminated, vinyl terminated and amino terminated. Such silicones have low tear strength and can be toughened by incorporating glass (SiO2) into the structure. Thus, an alkoxy functionalized siloxane can be included. Suitable alkoxy functionalized siloxanes include polydimethylsiloxane, tetraethoxy silane, tetramethoxy silane, and polydimethoxy siloxane.
One manner of forming the glassy matrix is using a Sol-Gel process employing an organotitanate compound, for example, a titanium alkoxide compound such as titanium methoxide, titanium ethoxide, titanium isopropoxide, titanium propoxide, titanium butoxide, titanium ethylhexoxide, titanium diisopropoxide (bis 2,4 pentanedionate), titanium diisopropoxide bis(ethylacetoacetate), or any other type of titanium alkoxide compound. These titanium alkoxide compounds can be used separately or in any combination. Although titanium alkoxides are given as examples, other organotitanate compounds can be used. The glassy matrix can also include a carboxylic acid compound. Silica gel is optional to inhibit the crosslinking reaction. Silica gel is used if storage over a long period of time is an issue. This is because it stores moisture. Alternatively, only silica gel can be used in place of the carboxylic acid compound. However, this does not work as well and a lot of silica gel is required.
With respect to the Sol-Gel process, as is well know to those of ordinary skill in the art, the Sol-Gel process is conventional, and typically produces a Sol-Gel glass which results from an optically transparent amorphous silica or silicate material produced by forming interconnections in a network of colloidal submicrometer particles under increasing viscosity until the network becomes completely rigid, with about one-half the density of glass.
In addition, the matrix can also include means for inhibiting or slowing the crosslinking reaction. Exemplary means for inhibiting include propionic or octonoic acid. Such agents permit the surface of the substrate to be coated to be painted before the formulation cures or crosslinks. The matrix can include various fillers and viscosity control agents such as mica, fumed silica, silica, kaolin, bentonite, zinc oxide, iron oxide, polytetrafluoroethylene powder, ultra high molecular weight polyethylene powder, high, medium and low molecular weight polyethylene powder, or other appropriate fillers, as will be readily apparent to those of ordinary skill in the art, can be used.
The two materials capable of microphase separation serve to prevent barnacles or other organisms from attaching or staying attached to the coating and thus the substrate. The material comprises at least one material, preferably a liquid, which in addition to its separation aspects, one of which is capable of being grafted into the glassy matrix. A material capable of microphase separation is a material that because of physical or chemical interactions between (among) the materials substantially continuously phase separates or moves. Such separation or movement in effect provides a substantially constant scraping motion thereby preventing the build up of slim or foul on the surface of the substrate.
One of the materials capable of microphase separation and graftable into the glassy matrix may be a vinyl terminated polydimethyl siloxane polymer reacted with dimethylethoxy silane and triethyl silane using a hydrosilylation reaction. Another microphase separatable material which can be employed is a methylhydrosiloxane polymer which is side-chain grafted with octene and vinyltriethoxy silane using a platinum-activated hydrosilylation reaction. Similarly, any hydrocarbon compound having a carbon atom chain of up to C20 can be used. Yet still further a methylhydrosiloxane polymer which is side-chain grafted with styrene and vinyltriethoxy silane using a platinum-activated hydrosilylation reaction can also be used. In this regard, it is noted that the platinum catalyst serves to react the hydride and the vinyl in the initial formulation. No platinum curing occurs in the final product provided in the field. Yet still further, a liquid hydrocarbon, such as polydecene, can also be used. In addition, other materials such as octanoic acid, heptanoic acid, hexanoic acid, decanoic acid, lauric acid, oleic acid, linoleic acid, linolenic acid, stearic acid, or any carboxylic acid containing a linear carbon chain of up to about C20 can be employed. Similarly, aluminum oleate, aluminum stearate, copper oleate, copper stearate, calcium stearate, magnesium stearate, or any fatty acid salt complex can be employed. In this regard, it is noted that of the above components, oleic acid is preferred as is aluminum oleate and aluminum stearate. Another microphase separatable material which can be employed is fluorotelomer intermediate available as Zonyl(copyright) from duPont.
The antifouling composition optionally may include an agent capable of preventing slime growth such as algae, bacteria, protozoa, diatoms and the like. Particularly preferred are surfactants and emulsifiers such as sorbitan mono- and tri-oleate (Span(copyright) products), polyoxyethylene sorbitan mono-oleate, polyoxyethylene sorbitan tri-oleate, and polyoxyethylene sorbitan mono-sterate (Tween(copyright) products), saponin, and cholesterol. Other suitable antifouling agents include silver compounds such as silver powder, silver oxide, silver chloride, silver carbonate, silver acetate and silver citrate; quaternary amine compounds such as cetrimide; isoniazid; benzalkonium chloride; sulfa-based antimocrobial compounds such as sulfanilamide, sulfaguanidine, sulfathiazole, sulfacetamide, sulfabenzamide and sulfamethiazole; saponin; and cholesterol. An exemplary antimicrobial compound is Irgarol(trademark), which is a slime inhibitor manufactured by, and commercially available from Ciba Geigy.
Additional additives which can be added to the formulation and can include aminopropyltriethoxy silane which serves to increase adhesion and can be used, for example, when the coating composition is used on the running gear of a vessel. Fumed silica can be used for thixotropic control, i.e., to increase viscosity. Pigments can be used to alter the color, and certain phosphates can also be added for anticorrosion properties.
The glassy matrix preferably comprises about 20 to 90 percent by weight of the composition; the liquid material capable of liquid phase separation preferably comprises about 1 to 30 percent by weight of the composition; and the agent capable of preventing slime preferably comprises about 0.1 to 25 percent by weight of the composition.
In operation, the antifouling composition of the present invention can be applied to a substrate by roll-coating, brush, spray coating dipping and the like. As discussed above, it is preferred that the user mix the catalyst with the other components right before or substantially contemporaneously with application to form the interpenetrating polymer network of glass and silicone on the surface of the substrate.